In mobile broadband cellular communication systems, there are several physical layer techniques that require a transmitter to be provided with knowledge of the channel response between the transmitter and a receiver. Transmission techniques that make use of the channel response between the transmitter and receiver are called closed-loop transmission techniques. One example of closed-loop transmission is the use of transmit precoding at the transmitter. An antenna array employing transmit precoding comprises of an array of multiple transmit antennas where the signals fed to each antenna are weighted in such a way as to control the characteristics of the transmitted signal energy according to some pre-defined optimization strategy, e.g. beamforming.
Generally, in such closed-loop multiple-input multiple-output (CL-MIMO) wireless communication systems, the transmitted antenna signals are weighted by applying weight vectors to multiple transmit antennas based on knowledge of the space-frequency channel response between each transmit antenna and each receive antenna. The transmitter uses these weight vectors and attempts to optimize the beamforming characteristics of the transmitted signal to be processed by the receiving device.
In those cases where a subscriber station (SS) or mobile station (MS) is located near the edge of a coverage cell of a base station (BS), where a signal strength is low and could be lost due to interference from a close neighboring cell, it has been envisioned that a neighboring BS can synchronize with a serving BS to provide additional coverage for this cell edge area and any MSs therein, and use beamforming to reduce interference for this MS. In particular, it is envisioned that downlink (DL) coordinated CL-MIMO is most useful for cell-edge users that are interfered by adjacent sector signals, and that a signal can be enhanced through DL beamforming from serving sector. In addition, interference can be avoided through DL beam nulling from adjacent sectors
One technique to control the transmit characteristics to this cell edge area is based on uplink feedback messages from a MS, such as can be obtained from an uplink control channel or uplink Channel Quality Indicator (CQI) channel, where the MS measures the channel response from the broadcast dedicated pilot signals for demodulation between the serving and neighboring BS antennas and the MS antennas, and transmits a feedback message back to one or both of the BSs containing enough information that enables the BS to perform closed loop transmit precoding. The BSs must then actively coordinate their transmissions to be synchronized. This requires an excessive amount of communication overhead, inasmuch as every transmission will require feedback from the MS to both BSs, the serving BS must set up the synchronization and beamforming parameters with the neighboring BS and must provide the message for the MS to the neighboring BS over a backhaul connection.
Accordingly, what is needed is a technique to provide passive coordination between base stations in order to alleviate the above described problems. In particular, a proposed solution should be supported with a regular backhaul connection and minimizing backhaul usage. In addition, it would be beneficial if a Signal-to-Interference plus Noise Ratio (SINR) is predictable, and if SINR feedback from a MS can match the real SINR with beamforming and interference nulling in order to improve scheduler efficiency. It would also be beneficial to reduce latency between scheduling and transmissions to a MS.
Skilled artisans will appreciate that common but well-understood elements that are useful or necessary in a commercially feasible embodiment are typically not depicted or described in order to facilitate a less obstructed view of these various embodiments of the present invention.